Currently, electrical insulation systems are made of anhydride-cured epoxies. The reason why electrical insulations are widely made from these materials is because they offer a good compromise between cost, electrical, mechanical and thermal properties. Anhydride-cured epoxies, however, present some important drawbacks especially from an environmental point of view. The use of anhydrides is under scrutiny of different authorities and could eventually be banned in the future. Anhydride-cured epoxies also are crack sensitive materials which causes problems in various manufacturing processes.
It has now been found that poly(butylene terephthalate) when made from a cyclic low molecular weight oligomeric poly(butylene terephthalate) resin can be used as an electrical insulation system and at the same time is an environmentally friendly recyclable material. Its excellent fracture toughness properties show the potential to decrease the occurrence of defects linked to matrix shrinkage upon cure. This material further offers new manufacturing possibilities which allow a decrease of production cycle time and, due to the non-exothermal polymerization reaction, also offers the possibility of manufacturing bulky parts with limited residual stresses.
Cyclic low molecular weight oligomeric poly(butylene terephthalate) resins have a low viscosity. Using such a cyclic low molecular weight oligomeric poly(butylene terephthalate) resin as a starting material has the advantage that, due to its low viscosity, it is possible to compound the oligomeric material up to 85% by weight with a filler material such as silica, which is of special importance for the manufacture of filled electrical insulation parts. This is not possible with known poly(butylene terephthalate). The resulting filler containing polymer made from the cyclic oligomeric compounds further has mechanical and electrical properties that cannot be matched with known filled poly(butylene terephthalate).
The low viscosity of the cyclic low molecular weight oligomeric poly(butylene terephthalate) resin gives also the possibility to manufacture large bulky electrical insulation parts with complex shapes, e.g. via injection molding, that cannot be manufactured directly with poly(butylene terephthalate). It further opens the possibility to impregnate reinforcements, like fiber fabrics or micatapes, which have been used in electrical insulation, which is not possible with known thermoplastics.